A variety of electromagnetic shutter mechanisms are available on the market today that are used for the control of the exposure of photographic film and/or solid state photosensor arrays. Leaf or blade type mechanical shutters have been used to regulate the amount of light allowed to reach the imaging area of the film or photosensor array as a function of the time that the shutter is rendered open (referred to as the "shutter speed"). The shutter opening size or aperture setting in cameras classically has been controlled by an adjustable iris diaphragm or, in less complex cameras, by the position of a further aperture blade having one or more sized apertures formed therein.
Alternatively, the shutter speed and aperture setting functions may be combined in a two blade mechanism (generally collectively referred to as a shutter) that is slidable between a fully closed position and a fully open position, having a number of settable opening positions defining apertures of various sizes, as shown, for example, in U.S. Pat. No. 4,171,897. In a further alternative system, the shutter may have one or more blades that are pivotal about one or more respective axes as shown, for example, in U.S. Pat. Nos. 4,265,530 and 4,658,230 and described in commonly assigned U.S. Pat. No. 5,150,149.
Typically, prior art shutters required the use of a mechanical spring and cocking mechanism set by the user advancing the film and released upon pressing the exposure button. In U.S. Pat. No. 4,671,638, a shuttering device is described that uses flexure springs to close and maintain closure of the shutter blades. Upon application of an electrical current to the coils of the aforementioned shutter, the shutter blades will open to a position determined by the spring force and amount of electric current supplied. Shortcomings of this type of shutter are that an additional component, the spring, is required, and shutter speeds are somewhat slower than if the spring was not incorporated in the design.
Alternatively, with the advent of motorized film advance, prior art shutter mechanisms have more recently utilized magnetic reluctance, or referred to here as a magnetic spring, as part of their function of setting the aperture and speed. See, for example, the above cited '149, '230, '530, and '897 patents and commonly assigned U.S. Pat. Nos. 4,881,093, 5,155,522 and 5,159,382, each incorporated by reference in their entireties, wherein shutter mechanisms are described that do not use mechanical springs to close the shutter blades. Rather, the phenomena of magnetic reluctance is used as a magnetic spring to close the shutter blades when current is removed from an armature coil.
In the above referenced, commonly assigned '149 patent, a single, rotatable permanent magnet is mounted to actuate a shutter blade and a plurality of aperture blades. The magnet rotates in one direction to set the aperture opening and then rotates in the opposite direction to actuate the shutter blade in response to the controlled reversal in polarity of the poles of a stationary electromagnet. A microprocessor based electronic control system provides control signals to energize the electromagnet in a fashion that sets the aperture as a function of a number of discrete pulses and controls the exposure duration or shutter speed. The system is complex, requiring encoding of the number of pulses for setting and resetting the selected aperture.
In the shutter mechanisms and actuators of the '522 and '382 patents, single magnets are mounted on axles to rotate from a stable rest position in relation to an unenergized electromagnet armature to first and second unstable or mono-stable positions under the influence of first and second direction magnetic fields generated by the armature in response to first and second direction currents. In the '522 patent, a shutter blade is directly mounted to the rotating magnet at its axle, and the blade rotates from its stable shuttering position to first and second aperture positions with respect to an imaging lens aperture. In the '382 patent, the single, bar shaped magnet is centrally mounted to an axle for rotation under the influence of the first and second direction magnetic fields, and the blade is pivotally mounted at a second fixed axle. Rotational movement of an end of the bar magnet is transferred to the blade to cause its rotation about its axis to present apertures to the fixed imaging lens aperture.
In the '093 patent, four rotating magnets and armatures are arranged to move four shutter blades in a moving iris arrangement to a desired aperture setting. On application of an electric current to the coils of these shutter blades, the shutter blade or blades open to a full open position, if the duration of the electric current is sufficiently long enough. When the electric current is removed, the shutter blades return to their fully closed position because of the magnetic spring effect. To achieve smaller than full open apertures, the electric current must be turned off or its direction reversed before the shutter blades reach a critical opening point at which the momentum of the shutter blades will carry the blades to their full open position. A shortcoming of this shutter is that discrete apertures are difficult to achieve due to the dynamics of the system.
In these actuators and shutter/aperture blade combinations, the single blade provides the shutter in its PG,5 stable, rest position when the armature coil is not energized. In the Cameo cameras sold by Eastman Kodak Co., one armature/magnet combination is dedicated for the aperture and is designed to provide two apertures, depending on the direction of current through the armature. Another armature/magnet combination is dedicated to the shutter and is designed to be either in a shutter closed position, in the absence of an energizing current, or moved to a shutter open position in response to an applied current. Only current passing in one direction will open the shutter blade. The benefits of this type of system are that multiple, discrete apertures are achievable for controlled exposure at rapid shutter speeds. The drawback of this type of shutter mechanism is that two armature/magnet combinations are required.
Dual armature, single rotating magnet actuators for a shutter/aperture blade mechanism are also described in Japanese Kokai Patent No. SHO 6311988]-36228. Simpler, single armature, single rotating magnet actuators are also described in Japanese Kokai Patent No. SHO 6311988]-36229.
As cameras are miniaturized and camera functions automated under the control of a battery powered electronic control system, compactness and low mass, low power consumption, high range and speed, and precision of the aperture setting of the shutter mechanism become more important. A need exists for a shutter mechanism that satisfies these requirements.